Hardwoods constitute over one-third of the U.S. timber resource. However, with the exception of cross-ties, hardwoods are rarely treated for exterior use applications, and demand for treated hardwood products has until recently been low. In view of the projected softwood timber shortage and relative abundance of hardwoods, expanded use of treated hardwoods for both composite and solid wood products is expected. The problem is that replacing softwoods with hardwoods is not straightforward since most wood preservatives, including second generation biocides, are considerably less effective when used to treat hardwoods. (Nicholas, D. D., Proc. of the Northern Hardwood Resource: Management and Potential Conference, Heighten, Mich., Aug. 18-20 (1986); Preston, A. F., et al., Proc. Am. Wood Preservers' Assn., 79, 207 (1983)). This disparity is attributable to the considerably higher toxic threshold values obtained when treated hardwoods are attacked by white- and soft-rot fungi as compared to softwoods treated with the same biocide and exposed to brown-rot fungi. (Nicholas, supra). Accordingly, the object of the present invention is to provide wood preservative systems that are effective in protecting hardwoods and providing greater efficacy for softwoods.
An apparent solution to the problem of preserving hardwoods is to use substantially higher biocide levels, but this approach leads to higher costs and increased environmental risks. A more attractive solution would be to increase the efficacy of biocides for treating hardwoods.
In addition, more environmentally benign preservatives to treat softwoods are needed, since all major wood preservatives used today to protect softwoods have perceived environmental problems.
Prior to the present invention, antioxidants have been used in wood treatments of various kinds, but never in conjunction with a biocide to generate a synergistic and environmentally safe preservative for both hardwoods and softwoods. In most previous wood preservatives containing antioxidants, their purpose was merely to stabilize the mixture from chemical decomposition or as a color stabilizer. For example, U.S. Pat. No. 1,168,062 discloses the use of oxidation inhibitors as an additive in oil-in-water emulsions for use in wood preservatives which contain pentachlorophenol as the active ingredient. U.S. Pat. No. 3,889,020 provides di-t-butyl cresol (also called butylated hydroxytoluene, or BHT) as a stabilizer for pentachlorophenol-based preservatives wherein the cresol is intended to improve the surface color of treated poles.
U.K. Patent Application GB 2,025,769A discloses the use of antioxidants, selected from such compound classes as sulfites, hydrosulfides, hydrazines and thiosemicarbazides. The purpose of the antioxidant there is to stabilize biocides from decomposition. U.S. Pat. No. 3,881,940 describes a composition containing an antioxidant stabilizer such as di-t-butylcresol in a biocide comprising a heavy metal oxide and pentachlorophenol. The antioxidant served to prevent discoloration of wood and to prevent sludge formation during treating steps.
U.S. Pat. No. 4,400,298 teaches the combination of dithiocarbamate and a borate with an antioxidant stabilizing agent, e.g., potassium metabisulfite, for the prevention of fungal decay in wood. U.S. Pat. No. 4,783,221 describes wood preservatives containing an isothiazolone and metal salts of carboxylic acids, to which various additives are added including antioxidants. U.S. Pat. No. 5,462,589 discloses a synergistic wood preservative composition comprising copper and organic derivatives, of which antioxidants are recited as possible additives.
Despite these teachings of antioxidants in wood treatment products, usually as stabilizers, synergistic enhancement of biocide effectiveness by antioxidants was unknown prior to the present invention. Moreover, this approach has never before been applied to provide an effective wood preservative for hardwoods.